The invention relates to a means for detecting gas, in which the light generated by discharge phenomena is radiated to a gaseous sample to ionize it and ions thereby generated are electrically detected. The photoionization detector is constructed in such a manner that ultraviolet light is generated by a discharge in the presence of a discharge gas; the light is guided into a detecting section to photoionize the sample gas; the sample gas ions are collected by a collector electrode in the detection section to measure the ionization current. Since a photoionization detector has excellent characteristics such as a high sensitivity of detection and a capability of detecting both inorganic and organic constituents, it is suited to a detector for gas chromatography.
As a prior art related to this invention, U.S. Pat. No. 4,028,617 is known. A means according to this patent comprises a discharge section which radiates light of electric discharge, a detecting section which collects an ion current generated by photoionization of a sample gas, and a light path which communicates the discharge section and the detecting section. A cathode for discharge has a tip directed to the light path. An anode for discharge forms a wall of the light path. Compared to a means known prior to the above U.S. patent, in which two bar electrodes are placed in a discharge chamber to obtain a spark discharge, the stability of discharge is improved and the base line variation was reduced to an order of 2.times.10.sup.-12 A. However, this value does not yet reach a value of variation 2.times.10.sup.-13 A, which is necessary for a practical use of a detector of this type.